We compared the ability of continuous-wave ultrasound to enhance cytotoxicity from X irradiation, hyperthermia or exposure to adriamycin. The survival of CHO cells exposed in culture medium to these agents was determined with and without continuous-wave ultrasound (1.62 or 1.765 MHz). In water-filled transmission exposure vessels with 2-cm-diameter Mylar end windows, 10-min insonation not producing cytotoxicity could produce .OH radicals (measured by electron paramagnetic resonance) even at 0.4 W/cm2. Ultrasound at intensities ranging between 1 and 2.5 W/cm2 increased the clonogenic cytotoxicity of adriamycin (P = 0.0023 by paired t test) but not of X rays (2-10 Gy) or hyperthermia (44 degrees C for 10-50 min). The only significant action of continuous-wave ultrasound under similar test conditions was the potentiation of adriamycin-induced clonogenic cytotoxicity, possibly mediated by cavitational activity.